1. Field of the Invention
This invention relates to a discharge tube, and more particularly to a discharge tube which is improved in stability in discharging.
2. Description of the Prior Art
Conventionally, discharge tubes wherein discharge gas is enclosed in an insulator tube and a voltage is applied between a pair of electrodes sealed at the opposite end portions of the insulator tube to cause discharge in the insulator tube are employed widely in various technical fields.
Referring to FIG. 13, there is shown an exemplary one of conventional discharge tubes. The discharge tube 1 includes a cylindrical envelope 2 of an insulating substance such as ceramic. A pair of metal layers 3 are formed on the opposite end faces of the envelope 2 by suitable means such as metallization. A pair of electrode plates 4 are sealed in an opposing relationship to each other to the opposite end faces of the envelope 2 with the metal layers 3 interposed therebetween. The electrode plates 4 have electrodes 5 formed from central portions thereof which project like bars toward each other. Discharge gas such as argon gas is enclosed in the internal spacing of the envelope 2. Upon manufacture of the discharge tube 1, the discharge gas is introduced into the discharge tube 1 by way of an enclosure pipe 6 securely mounted on a left-hand side one in FIG. 13 of the electrodes 4 and an enclosure hole 7 formed in the adjacent electrode 5, whereafter the enclosure pipe 6 is closed. During use of the discharge tube 1, a predetermined sufficiently high voltage is applied between the electrode plates 4, and thereupon, discharge takes place in a discharge gap G between the inner opposing end portions of the electrodes 5.
In the conventional discharge tube 1, the bar-like electrodes 5 are positioned within the envelope 2, and the end portion of each of the electrodes 5 has a cross section of a generally semicircular shape as seen in FIG. 13. With the bar-like electrodes 5 of such configuration, the electric field around the surfaces thereof and in the discharge gap G is liable to become a non-uniform electric field which varies in intensity at various locations. Consequently, the starting voltage fluctuates significantly, for example, 20 to 30% respect to the average value thereof. Also the aging is great. Accordingly, the conventional discharge tube has a problem that discharge is not stable. It is another problem of the conventional discharge tube that, if a neighboring conductor having a certain potential is positioned in the proximity of the envelope 2 of the discharge tube 1, then the discharge tube 1 is influenced significantly by an electric field produced by the neighboring conductor so that the starting voltage of the discharge tube 1 is fluctuated significantly. The conventional discharge tube 1 has a further problem that, when a saw-tooth voltage is applied, the starting time is fluctuated by the saw-tooth voltage and discharge is not stabilized.
One of conventional solution to the problems is to form an electrode into a uniform electric field forming electrode which has a specific sectional shape at an end portion thereof such that the central portion thereof is formed as a flat face while a peripheral edge portion is formed as a curved face such as a Rogowski electrode, a Bruce electrode or a Harrison electrode so that the electric field around the surfaces of the electrodes and in the discharge gap is approximated to a uniform electric field in order to stabilize discharge.
However, in order to form an electrode into a uniform electric field forming electrode such as a Rogowski electrode, the electrode must have a large diameter equal to three to seven times the dimension of the discharge gap. This results in the drawback that the diametrical dimension of the entire discharge tube is very large. Further, such a uniform electric field forming electrode described above requires a precision working technique for surface working and so forth. Accordingly, there is another drawback that the manufacture of the uniform electric field forming electrode is very difficult and the production cost is very high.
Further, even if each electrode is formed into a uniform electric field forming electrode, since the entire electrodes are positioned in the inside of the envelope, if a little working error takes place in surface working, then the electric field becomes stronger at the location and discharge takes place from the portion. Accordingly, there is the possibility that discharge may be unstable.